In hydroprocessing, which may include hydrotreating, hydrofinishing, hydrorefining, and hydrocracking, a catalyst is used for reacting hydrogen with hydrocarbons, such as a petroleum fraction, distillates, resids, or other hydrocarbon compounds, for the purpose of saturating olefins or removing heteroatoms, such sulfur, nitrogen, oxygen, metals, or other contaminants, or for molecular weight reduction (cracking). Catalysts having special surface properties are required in order to provide the necessary activity to accomplish the desired reaction(s).
In conventional hydroprocessing it is necessary to transfer hydrogen from a vapor phase into the liquid phase where it is available to react with a hydrocarbon molecule at the surface of the catalyst. This is accomplished by circulating very large volumes of hydrogen gas and the liquid hydrocarbon feed through a catalyst bed. The liquid feed and the hydrogen flow through the bed and the hydrogen is absorbed into a thin film of liquid hydrocarbon that is distributed over the catalyst. Because the amount of hydrogen gas required for the hydroprocessing reaction can be quite large, e.g., 1000 to 5000 SCF/bbl (0.178 to 0.890 m3/L) of liquid, the reactors are very large and are operated at severe conditions, from a few hundred psi to as much as 5000 psi (34.5 MPa), and temperatures from around 400° F.-900° F. (204° C.-482° C.). Furthermore, because conventional processes move very large quantities of hydrogen gas through the reactor, they also require the use of very large external high-pressure separators to recover gas from the product stream. These high-pressure separators may be as large as the hydroprocessing reactors and are a significant capital equipment cost.
Hydroprocessing rates are typically measured in terms of mass flux, which can be defined as the mass flow rate per unit area. In a reactor the mass flux is the mass flow rate through a reactor divided by the cross-sectional area of the reactor. Typical mass flux in hydroprocessing reactors fall in the range of from 1,000 lb/hr·ft2 (4880 kg/hr·m2) to less than 5,000 lb/hr·ft2 (24,400 kg/hr·m2). Although it is preferable to move liquid feedstocks through the reactor at the greatest rate and volume as possible (i.e., a higher mass flux), a variety of limitations, including excessive pressure drop, hydrogen mass transfer concerns, liquid holdup, and wetting inefficiencies, have necessitated that mass flux rates remain within the range of from 1,000 lb/hr·ft2 (4880 kg/hr·m2) to less than 5,000 lb/hr·ft2 (24,400 kg/hr·m2) for optimum process efficiency.
A method for hydroprocessing is disclosed in U.S. Pat. No. 4,937,051, entitled Catalytic Reactor with Liquid Recycle, issued to Graven et al. on Jun. 26, 1990. Graven et al. discloses a continuous catalytic reactor column for contacting oil and a treating gas in a multiphase fixed bed catalytic reactor having at least two operatively connected catalyst beds . . . and including means for maintaining total liquid flux in at least one catalyst bed at a rate of about 2000 lb/hr·ft2 (9760 kg/hr·m2). Graven et al. also states that, “[i]n a typical multi-phase reactor system, the average gas-liquid volume ratio in the catalyst zone is about 1:4 to 20:1 under process conditions. Preferably the liquid is supplied to the catalyst bed at a rate to occupy about 10 to 50% of the void volume.”
A typical range for mass flux in a hydroprocessing reactor is disclosed in U.S. Pat. No. 7,655,135, entitled Process for Removing Solid Particles from a Hydroprocessing Feed, issued to Havlik et al. on Feb. 2, 2010. Havlik, et al., discloses a process for removing inorganic solid contaminants 10 microns and smaller from a hydroprocessing feed stream wherein lower mass fluxes are expected to allow a higher utilization factor due to lower velocities which promotes solids laydown in a guard bed that has the purpose of trapping solids before a feed is introduced into a typical hydroprocessing reactor. Havlik et al. suggests a preferred mass flux of 500 lb/hr·ft2 (2440 kg/hr·m2) and a more preferred mass flux of 1,000 lb/hr·ft2 (4880 kg/hr·m2) for the guard bed in the disclosed invention. Havlik et al. goes on to provide an example of a 20,000 bbl/day (3179 m3/day) gas-to-liquids plant wherein the guard bed operates at a mass flux of half of the mass flux of the subsequent hydrocracker, which is disclosed as being as 1500 lb/hr·ft2 (7320 kg/hr·m2).
U.S. Pat. No. 6,569,313, entitled Integrated Lubricant Upgrading Process, issued to Carroll et al. provides further support of the typically accepted mass flux in catalytic hydroprocessing. It is described therein that in the preferred embodiment the catalytic dewaxing segment of the process advantageously, the liquid flux rate for total feed rate (including optional liquid recycle) is maintained in the range of 500-3500 lb/hr·ft2 (2440-17,100 kg/hr·m2), preferably 1000-3000 lb/hr·ft2 (4880-14,600 kg/hr·m2).
In U.S. Pat. App. Pub. No. 2012/0074038, entitled Liquid Phase Hydroprocessing with Low Pressure Drop, of Petri et al., a process is disclosed wherein the mass flux in a hydroprocessing reactor may be in excess of 6000 lb/hr·ft2 (29,300 kg/hr·m2). This process is limited in that the average size of the catalyst particles utilized must be in the range of 100 nm-1.27 mm. As stated therein, larger catalyst sizes require that the mass flux of the reactor be reduced.